FERONIA phosphorylates the amino-terminal extension of phytochrome B to modulate plant light and temperature responses

FERONIA (FER) receptor kinase is a critical regulator in balancing plant growth and stress responses. As an active kinase, FER phosphorylates many proteins to regulate their stability, nuclear accumulation, and condensation in diverse biological processes. Phytochrome B (phyB) is a thermosensitive red/far-red photoreceptor that can switch between an inactive Pr and an active Pfr conformer via light-dependent interconversion and temperature-dependent Pfr-to-Pr thermal reversion (Pfr-to-Pr). The phyB N-terminal extension (NTE, aa1–90) plays an essential role in stabilizing Pfr, and NTE phosphorylation at multiple sites can decrease Pfr stability, thereby serving as a critical control for light sensitivity. However, direct experimental demonstration of kinase(s) responsible for NTE phosphorylation and their site specificity is lacking. Here we show that FER phosphorylates Ser24 and Ser25 of phyB NTE. Genetic analysis demonstrates that FER modulates phyB-mediated responses to red light and temperature. While the fer-4 mutant is hypersensitive to red light-inhibited hypocotyl growth and less sensitive to warm temperature-induced hypocotyl elongation, the fer-4 phyB-9 double mutant largely mimics phyB-9 , supporting a function of FER to tune down phyB activity via NTE phosphorylation. Consistently, phosphosite mutational analysis showed that phosphorylation at Ser24 and Ser25 destabilizes phyB Pfr by accelerating thermal reversion and reduces phyB photobody formation and signaling output. Together, these results reveal a direct link between FER and phyB, in which FER phosphorylates Ser24 and Ser25 of phyB NTE to fine-tune light and temperature responses.